Note: Descriptions are shown in the official language in which they were submitted.
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Heat exchanger
The invention relates to a heat exchanger for transferring heat
from a first fluid to a second fluid, comprising one or more
flow passages for a first fluid, which are arranged parallel to
and at a distance from one another and the outer wall of which
is in heat-transferring contact with a flow body for a second
fluid, which is made from metal foam.
EP-A-0 744 586 has disclosed a heat-transfer element, for
example a plate or tube, with a large heat-transferring surface
in the form of copper foam, for use in a heat exchanger, in
order to improve the heat transfer. An element of this type is
produced by using a vapour deposition process to deposit a
powder of copper oxide on a plastic foam which has previously
been provided with a suitable adhesive. The foam which has been
prepared in this way is then arranged under slight pressure on a
plate or tube, which has likewise previously been covered with a
copper oxide powder, in order in this way to form a composite
element by sintering. After pyrolysis of the plastic foam, the
copper oxide is reduced to form copper.
A heat exchanger of the type described above is used, for
example, in what are known as thermo-acoustic heat engines. In a
heat exchanger of this type, a first heat circuit is formed by a
flow of a first fluid, such as a gas or liquid, through
generally a plurality of flow passages. A second heat circuit
comprises a flow of a second fluid, generally a gas (air,
argon), through the porous flow body, which flow body surrounds
the flow passages over a certain area. The direction~of flow of
the second fluid through the flow body is generally virtually
perpendicular to the direction of flow of the first fluid in the
flow passages. The porous flow body is in heat-exchanging
contact with the outer wall of the flow passages. Heat is
transferred, for example, from the first fluid to the inner wall
of the flow passages and is carried to the outer wall as a
result of conduction in the wall material. At the outer wall,
heat transfer to the porous flow body takes place through
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radiation and conduction. Heat conduction takes place in the
porous flow body. When there is only a flow body made from metal
foam, this heat conduction is limited, and consequently solid
lamellae made from a material with good conductivity are
sometimes provided in the metal foam in order to increase the
heat conduction. Transfer of heat from the flow body to the
second fluid likewise takes place by means of radiation and
conduction. The efficiency of the heat transfer overall is
dependent, inter alia, on all these transitions, the transfer
from the flow body to the second fluid or vice versa - generally
the heat transfer on the gas side - in particular possibly
representing an inhibiting factor.
It has now been found that, although the use of a metal foam,
optionally in combination with lamellae or fins, offers an
enlarged heat-exchanging surface area and possibly increased
conduction, the flow resistance is relatively high, so that the
overall performance, expressed as the ratio between heat
transfer and flow resistance, is inferior to that of a
conventional heat exchanger with only fins or lamellae. In many
cases, an increase in the heat transfer when using a metal foam
goes hand-in-hand with a disproportionate increase in the flow
resistance.
US-A-4,245,469 has disclosed a heat exchanger in which a porous
metal matrix is arranged in a flow passage through which a heat-
transferring medium flows. It is stated that this metal matrix
has a greater density in an area which is perpendicular to the
direction of flow, so that the internal heat transfer
coefficient is increased in this area, where the temperature of
the environment is much higher than at the end of the passage .
To minimize the reduction in volume of the heat-transfer medium
which would be produced with a passage of constant diameter, the
diameter is increased at the location of the said area. A design
of this type aims to improve the internal heat transfer.
Furthermore, DE A1 39 06 446 has disclosed a heat exchanger in
which a foam, for example of aluminium, is arranged in a flow
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passage. If desired, the pore size in this foam may be varied,
i.e. the number of pores may vary.
The general object of the invention is to improve the overall
performance, i.e. the abovementioned relationship between heat
transfer and flow resistance, of a heat exchanger.
In the heat exchanger of the type described above, according to
the invention the metal foam has a gradient of the volume
ZO density of the metal. The use of a metal foam with a gradient of
the volume density enables the volume density of the foam, in
other words the amount of metal, to be adapted to the local heat
flux density and flow resistance, while the number of pores
(PPT) remains the same. In the metal foam, the heat flux density
l5 is highest in the vicinity of the flow passages, so that the
metal foam should contain more metal at this location than at
the outer periphery of the flow body, where the heat flux
density is much lower. This is possible as a result of the
volume density of the metal of the metal foam used being varied.
20 The arrangement of the metal foam in the heat exchanger
according to the invention has the object of promoting the heat
transfer from the metal foam to the wall of the flow passage. A
volume gradient of the metal in the metal foam while the PPI
remains identical is more effective than varying the number of
25 pores while the thickness of the metal webs which separate the
pores remains the same.
Metal foam with a gradient of the volume density of this type
can be obtained, for example, by electroplating methods for the
30 electroplating of a plastic foam in an electrolysis bath, as
will be explained in more detail below.
It should be noted that FR-A-2 766 967 has disclosed a neat
sink, inter alia for electronic components, which comprises a
35 metal foam with a gradient of the thickness of the deposited
metal in the thickness direction of the foam.
Since in a production method of this type the density in the
foam changes in one direction, the flow body preferably
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comprises at least two layers of metal foam, of which layer
surfaces which have the same volume density face towards one
another. This allows various advantageous embodiments of the
flow body to be achieved.
In a first preferred embodiment, the volume density of the metal
foam increases from an inflow side of the flow body for the
second fluid towards a flow passage, so that more metal is
present where the heat flux density is greater.
The shape of the flow passages is not critical round tubes,
flat hollow plates and the like can be used. However, to limit
the flow resistance, the shape of a flow passage is preferably
adapted to the flow profile of the second fluid. A flow passage
advantageously has an elliptical cross section, the main axis of
which extends in the direction of flow of the second fluid. A
flow passage of such a shape combine s a large heat-exchanging
surface area with a relatively low flow resistance.
The flow body then advantageously comprises two layers of metal
foam, preferably having the same number of pores per linear inch
(PPI), of which the sides with the highest metal volume density
face towards one another. In those sides, recesses for the flow
passages are provided.
According to another preferred embodiment, which is advantageous
in particular on account of the simple modular structure, the
flow passages comprise tubular bodies which are rectangular in
cross section and are separated by sections of the flow body,
the volume density of the sections of the flow body being
highest in the vicinity~'of the outer walls of the flow passages.
A module of this preferred embodiment of a heat exchanger may
comprise, for example, a flow passage of this type which is
rectangular in cross section and of which two opposite walls are
provided with a layer of metal foam, of which the layer surface
with the highest volume density adjoins the walls in question.
If a heat exchanger which more closely resembles a heat
exchanger with a flow body comprising metal foam parts separated
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by lamellae is desired, it is possible to use a plurality of
layers of metal foam, of which the gradients of the volume
density run parallel to the direction of flow of the first
fluid, preferably alternately. In terms of overall performance,
this embodiment is less preferred than the other variants
described above.
If a metal foam is selected as material for the porous flow
body, the heat transfer between metal foam, on the one hand, and
the second fluid, on the other hand, is high and no longer the
limiting factor, on account of the very large heat-exchanging
surface area for a given volume.
The heat conduction in the flow body made from metal foam,
however, is low, on account of the porosity thereof, which
porosity also has an adverse effect on the heat transfer between
the flow body and the outer wall of the flow passages. A gradual
increase in the quantity of metal in the foam leads to an
improvement in the overall effect of these two contradictory
factors.
It is preferable to use a metal foam made from a metal with a
high heat conduction coefficient, such as copper. The flow
bodies are advantageously also made from a metal with high heat
conduction and heat transfer, such as copper. Other suitable
metals include, inter alia, indium, silver, nickel and stainless
steel. The starting material used for the production of the
metal foam is advantageously a plastic foam, such as
polyurethane, polyester or polyether with an open network of
interconnected pores and a constant PPI value. The diameter of
the pores is preferably in the range from 400-1500 micrometers,
more preferably 800-1200 micrometers. The volume gradient may
rise from less than 5o to more than 95o in the direction of flow
of the fluid flowing through the~foam. The thickness of the
metal deposited on the plastic foam advantageously has a
gradient which ranges from 5-10 micrometers, preferably at the
inflow side of the flow body, to 30-70 micrometers, preferably
in the vicinity of the flow passages, for example 8 micrometers
and 42 micrometers, respectively. Metal foams of this type are
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easy to produce by means of electroforming of, for example,
copper on a substrate of polymer foam in a suitable electrolysis
bath, optionally followed by pyrolysis of the polymer. If
desired, a thin conductive layer, fox example a copper layer,
may first be deposited on the foam using other techniques, for
example (magnetron) PVD, CVD and the like, after which this film
is allowed to grow further in the electrolysis bath.
Various welding techniques (induction, diffusion) and soldering
techniques can be used to attach the metal foam to the flow
passages. Tin-containing soldering alloys are eminently suitable
for copper foam.
The heat exchanger according to the invention is preferably of
modular structure, so that a plurality of modules can be
combined to form a larger unit.
The invention also relates to a heat pump, for example a thermo-
acoustic conversion device, for converting energy as defined in
claim 11, in which heat exchangers according to the invention
are used. The motor for compressing and displacing the gaseous
fluid is, for example, a closed acoustic resonance circuit. The
regenerator used preferably has a layered structure comprising
foam layers of a metal with poor conductivity. Examples of a
thermo-acoustic conversion device of this type include a thermo-
acoustic heat engine and a thermo-acoustic motor.
The invention will be explained below with reference to the
appended drawing, in which:
Figure 1 shows a perspective view of an embodiment of a heat
exchanger according to the prior art;
Figure 2 shows a perspective view of a first embodiment of a
heat exchanger according to the invention;
Figure 3 shows a perspective view of a second embodiment of a
heat exchanger according to the invention;
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Figure 4 shows a perspective view of a module of the heat
exchanger according to claim 3;
Figure 5 shows a perspective view of a third embodiment of a
heat exchanger according to the invention; and
Figure 6 diagrammatically depicts a thermo-acoustic conversion
device for energy conversion, in which heat exchangers according
to the invention are used.
In the embodiment of a heat exchanger 10 according to the prior
art which is illustrated in Figure 1, a number of tubular flow
passages 12, for example made from copper, are arranged parallel
to one another. The direction of flow of a first fluid through
the flow passages 12 is indicated by a single arrow, in the
situation illustrated from the top downwards . The inlet ends 14
of the flow passages 12 are usually connected to one another
with the aid of a distributor cap (not shown). The outlet ends
16 are connected to one another in a similar way. A porous flow
body for a second fluid is denoted overall by reference numeral
20 and comprises a number of metal strips 22 which are arranged
at a distance from and parallel to one another and each have a
layer 24 of metal foam between them. Holes for the flow passages
12 are provided at the appropriate locations in the metal strips
22 and layers 24. The metal strips 22 are soldered to the outer
walls 26 of the flow passages 12. The flow body 20 is arranged
in a chamber or housing (not shown), which are provided with a
feed and a discharge and, if desired, distributor means for the
second fluid. The sides of the housing of the heat exchanger 10
may be provided with coupling means, so that a plurality of heat
exchangers can be coupled to one another as required.
Figure 2 shows a preferred embodiment of a heat exchanger
according to the invention, in which identical components to
those shown in Figure 1 are denoted by the same numbers and
references.
The heat exchanger 10 comprises a number of parallel flow
passages 12 which are arranged at a distance from one another
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and have an elliptical cross section, through which a first
fluid, for example a liquid, is guided. The flow body 20
comprises two metal foam parts 30 and 32, each with a gradient
of the volume density parallel to the direction of flow of the
second fluid, for example a gas. To simplify the figure, the
surface with the highest volume density is indicated by a thick
solid line in this figure and the following figures. In part 30,
the volume density (amount of metal) increases in the direction
of flow of the second fluid, while in part 32 the volume density
decreases in the direction of flow indicated. Consequently, most
metal is present in the immediate vicinity of the flow passages
12, where the highest heat flux density also prevails. The outer
surface of the flow body 20, in particular the inflow side (and
discharge side), is relatively open.
Figure 3 shows another embodiment, in which flow passages 12
which are rectangular in cross section are arranged between
sections 40 of the flow body 20. Each section 40 is composed of
two metal foam layers 42, whose surfaces with the highest volume
density adjoin the outer walls 44 of two flow passages 12
arranged next to one another, while the surfaces having the
lowest volume density bear against one another. In this figure,
the separating surface between the two foam layers 42 of a
section 40 are indicated by a dot-dashed line. Figure 4 shows a
module of the embodiment of a heat exchanger according to the
invention illustrated in Figure 3.
Figure 5 shows yet another variant of a heat exchanger according
to the invention, in which six alternately stacked metal foam
layers 50 are provided as flow body 20, the gradient of which
alternately increases and decreases repeatedly as seen in the
direction of flow of the first fluid which is guided through the
flow passages 12.
Figure 6 shows an outline sketch of a heat pump according to the
invention, in this case an embodiment of a thermo-acoustic
conversion device 60 for energy conversion, in which heat
exchangers according to the invention can advantageously be
used.
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The device 60 comprises a gas-filled acoustic or acousto-
mechanical resonance circuit 62 with a regenerator 64, for
example made from nickel foam, arranged between two heat
exchangers 10 according to the invention. If the device 60 is
used as a heat pump, mechanical energy is supplied to the gas,
for example via a diaphragm which is made to oscillate with the
aid of a linear electric motor. Other possibilities include, for
example, a bellows or a free piston structure. The gas which has
been made to oscillate and functions as a second fluid extracts
heat from a first fluid in the first heat exchanger 10 and pumps
the extracted heat via the regenerator to the second heat
exchanger 10, where the heat is transferred to a third fluid. In
this way, it is possible to transfer heat from a flow of fluid
which is at a low temperature to a fluid which is at a high
temperature. The periodic pressure variation and gas
displacement required for this process takes place in the closed
resonance circuit 62 under the influence of a powerful acoustic
wave. At this point, it should be noted that the pressure
amplitude is many times greater than is customary in a free
space, namely of the order of magnitude of 100 of the mean
pressure in the system.
If the conversion device is used as a motor, heat is supplied to
a heat exchanger at high temperature and is dissipated by a
further heat exchanger at low temperature, for example ambient
temperature, with the result that the oscillation is maintained.
If more heat is supplied than is necessary to maintain the
oscillation, it is possible for some of the acoustic energy to
be extracted from the resonator as useful output.
The performance of the heat exchangers according to the
invention is explained in more detail below on the basis of the
following examples.
Various heat exchangers were produced and tested. The porous
flow body of a first heat exchanger A is made from strips of
copper foam (65 pores per inch) with a length of 90 mm and a
width of 12 mm. Holes are stamped out for the flow passages. The
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flow passages comprised nine small copper tubes, with an
external diameter of 6 mm (internal diameter 4 mm) arranged at
regular intervals. The effective passage for the second fluid is
90 mm x 70 mm. Manifolds at the inlet ends and outlet ends of
the small copper tubes were connected to a water feed and a
water discharge, respectively.
In a second heat exchanger B, a flow body made from the same
copper foam is used, but brass lamellae with a thickness of
0.25 mm are fitted in this heat exchanger. The foam and the
lamellae are soldered together in a furnace. To prevent the
metal foam from closing up under the influence of heat, the
strips of copper foam and brass lamellae can also be soldered
one by one to the small copper tubes.
In a third heat exchanger C, the flow body only comprises 39
brass lamellae.
In a fourth heat exchanger D according to the invention, as
shown in Figure 2, having the same dimensions and number of
tubes~as heat exchangers A-C, the flow body comprises two layers
of copper foam, which were produced at room temperature on a PU
foam with a pore diameter of 800 micrometers in a copper bath of
composition CuS04 = 250 g/1, H~S04 = 70 g/1, Cl- - 15 mg/1 and pH
- 0-1, at a current density of 5 A/dm2. After pyrolysis, a copper
foam layer produced in this way had a metal thickness of
8 micrometers on one side, while on the other side the thickness
of the deposited metal was 42 micrometers. Recesses
corresponding to half the diameter of the small copper tubes
were provided in the latter sides of these foam layers, after
which the small tubes were positioned in these recesses. Tin
soldering was used as the joining technique.
These heat exchangers were used to carry out tests, in which a
quantity of hot water (T - approx. 80°C) controlled using a
flowmeter was circulated through the small tubes via a
thermostat bath. A centrifugal pump was used to suck ambient air
through the flow body of the heat exchanger, which was arranged
in a passage. The volume of air sucked in was measured using a
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flowmeter between the heat exchanger and the centrifugal pump.
The pressure drop across the flow body and the inlet temperature
T1 and outlet temperature Ta of the first flow of fluid,
comprising water, and the outlet temperature T3 of the second
flow of fluid, comprising air, were measured. The quantity of
heat Q absorbed by the flow of air is calculated from the
volumetric flow rate of water FW (1/min) and the temperature
difference between the incoming and outgoing flow of water (T1-
T2) using the following formula:
Q = Ww. (T1-TZ) . FW/60 [W] ,
where Ww is the heat capacity of water (4180 J.Kg.K-1). The tests
were carried out at various air velocities. The Reynolds number
was determined from the measured gas velocity at the location of
the heat exchanger and the hydraulic diameter DH=0.0033 for all
the heat exchangers A-D. The viscosity value applies at the gas
temperature of the fresh air sucked in, which temperature was
likewise measured. The Nusselt number for the gas side can be
calculated by eliminating the heat transfer on the liquid side
and assuming turbulent tube flow: Nu (Re) - Q. DH/~,.OT1, where AW
is the total heat exchange surface area and OT1 is the
temperature difference between gas and heat exchanger.
As is customary in the specialist field, the heat transfer is
represented as jH - Nu.Re-l.Pr-1~3 against Re, where Pr is the
Prandtl number, which for air is 0.7.
The so-called friction coefficient can be calculated in the same
way
f = Ao 4p /AW ( 1 / 2 p v2 )
from the measured pressure drop and the measured velocity for
these heat exchangers of known dimensions and can be represented
as a function of the Reynolds number.
The table below shows the results of the heat transfer (jH), the
friction coefficient (f) and the ratio jH/f for Re=300 for the
various heat exchangers A-D.
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Table
Heat exchanger jH f jH/f
A 0.07 20 0.004
B 0.7 40 0.018
C 0.03 1.4 0.021
D 0.5 15 0.033
It can be seen from the above table that, as expected, heat
exchanger A (foam alone) provides a higher heat transfer than
heat exchanger C (lamellae alone). However, the flow resistance
has increased disproportionately. Furthermore, it can be seen
that, although heat exchanger B (foam and lamellae) achieves a
higher heat transfer than heat exchanger D according to the
invention, the flow resistance is very high. The heat. exchanger
according to the invention has the best overall performance,
expressed as jH/f. It is clear from this that, by using a foam
with a suitable distribution of metal and by changing the amount
of this metal, it is possible to achieve a favourable balance
between heat transfer/conduction, on the one hand, and flow
resistance, on the other hand.
